I bought this bag because it locks into the blackburn locking rack, provides nice every day storage and provides expandability with drop down panniers for that spur of the moment purchase while on a bike trip. This bag does all of this.
Unfortunately, the bag flops to one side, occasionally rubbing against the tire. One solution is to find or make and insert a cardboard box the dimensions of the main compartment. This will hold it up somewhat firm. Another solution is to first note that the security straps are way to loose for the locking panel of Blackburn's locking rack. Then use the tightening straps, not around the full rack, but around the locking panel and pull tight. The velcro won't line up, so find an innovative way to fasten. The best solution is to use both of these methods.
Still, at about 40% of the cost of Topeak's similar bag, which is probably not lockable, it is a great bargain. But buyer beware.